Epoxy group titration

Another example of interest with regard to the reaction mechanism is the analysis of epoxy groups. Durbetaki60 titrated a-epoxy compounds with HBr (cf., p. 260) in glacial acetic acid with crystal violet as indicator, but the method was slow for glycidyl esters, CH2—CHCH2OOCR. As it concerns a two-step... 

Analytically, epoxy groups are determined by the reaction with hydrogen halide and back titration with a standard base. Other functional groups present may cause interference problems and result in poor end points. Pyridinium chloride-pyridine is a recommended reagent for the analysis of bisphenol-diglycidyl ether resins [22,23],... 

Epoxy content is generally determined by wet analytical techniques. ASTM D 1652 is widely used. The most common method is based on the addition of hydrogen halide (e.g., hydrogen chloride, hydrogen bromide, or hydrogen iodide) to the epoxy group. The difference between the amount of acid added and the amount unconsumed, determined by titration with standard base, is a measure of the epoxy content. There are numerous epoxy assay methods based on this technique. The specific method used depends on the resin type being analyzed as well as on the extent to which side reactions can occur. 

Aromatic amines and epoxy groups may be determined by titration when present in the same sample. The former is carried out with 0.1 M of HC104 in glacial acetic acid, using Crystal Violet (150) as indicator, whereas the latter requires epoxide opening with HC1 and the presence of tetraethylammonium bromide250. 

Various properties of the obtained alkyds are displayed in Table 3. Basically, the finished alkyds have three fundamental functional groups, that is hydroxyl, carboxylic and olefinic groups. It was postulated that oidy the hydroxyl and carboxylic groups in alkyds might react with the epoxy groups of ENR at ambient temperature. Hence, it was important to determine the free hydroxyl and carboxylic groups remained in those alkyds prior to the manufacture of unvulcanized mbber composition. Both the free hydroxyl and carboxylic groups could be determined from the hydroxyl number and acid number titrations, respectively. 

Epoxy resins are characterised by two or more epoxy groups in the structure. To cure an epoxy resin with a suitable hardener, accurate estimation of epoxy groups is crucial. Epoxy equivalent is defined as the amount of resin that contains one mole of epoxy. Epoxy equivalent of an epoxy sample is determined by a standard titration method [6] using hydrogen bromide solution in acetic acid. The method is briefly described next. 

Determinations of epoxy groups. Epoxy groups are usually titrated by Durbetaki s method or by Jay s derivative process In the latter, which today is the most commonly used, epoxy groups are reacted with HBr resulting from the reaction of perchloric acid with tetraethylammonium bromide. This reaction, which is stoichiometric, is carried out in the presence of crystal violet as indicator. When all epoxy groups have reacted, the excess of HBr changes the colour of the indicator. 

In another procedure, a halogen acid is generated by the reaction of an ionic halide salt, eg, tetraethylammoniiun bromide in acetic acid with perchloric acid with subsequent formation of a halohydrin the epoxy group is determined by back-titration with perchloric acid nsing crystal violet indicator (83). The end point can be determined visually or potentiometrically. A monograph on epoxide determinations was published in 1969 (84). This is the method adopted by ASTM and is currently used by most resin producers. 

If Q is defined as 1/WPE, then Q represents the number of available epoxy functional groups in a given mass of the epoxy resin. Qo (= 1/WPEq) represents the number of epoxy groups initially available in an unreacted sample and is easily obtained from a titration of the pure ECN. Consequently, the fraction of epoxy unreacted is... 

Epoxy groups Epoxy resins Titration with halogen acid spectrophotometric 189... 

The amount of silane and triethylamine used should be approximately proportional to the surface area of the silica thus, if a silica of 50 m /g is coated then five times less reagents should be used. The epoxy group concentration on coated silica can be determined by titration of the hydroxyl ions released during the reaction of the epoxy groups with thiosulphate (4). To do this suspend up to 100 mg of dry epoxy-silica in 2 ml water and adjust the pH to 7.0. Add 1 ml of a 3 M solution of sodium thiosulphate, pH 7. 0, to initiate the reaction. During the course of the reaction ( — 60 min) maintain the pH at 7.0 by adding 0.1 M HCl. The consumption of HCl corresponds to the epoxy group content of the gel. 

Amino-terminated telechelic polybutadiene was prepared by LiAlH4 reduction of amidino end-group in polybutadiene, which was polymerised by a water-soluble initiator, 2,2 -azobis(amidinopropane)dihydrochloride. The structure was analysed by 1H- and 13C-NMR, but functionality of 2.0 was obtained by a titration method [70]. Synthesis of co-epoxy-functionalised polyisoprene was carried out by the reaction of 2-bromoethyloxirane with living polymer that was initiated with sec-butyl lithium. The functionality of the resulting polyisoprene was 1.04 by 1H-NMR and 1.00 by thin layer chromatography detected with flame ionisation detection [71]. 

Degree of cure. The titration results given in Table III reveal that from 98% to 100% of the functional groups had reacted. Similarly no evidence for incomplete cure was observed by DSC or by dynamic mechanical spectroscopy (DMS). However, it may be pointed out that 2% unreacted functional groups could result in detectable incoherence (see reference 29) in the networks prepared from high-MW epoxy prepolymers. 

This cure cycle was reported by Bell to give essentially complete curing, and was also shown (through chemical titration of the epoxy and amine groups) to be similarly effective in this laboratory ( 5, 2). ... 

An additional way to produce a secondary amine group utilized the reaction of 8 moles of piperazine with 1 mole of the epoxy terminated siloxane oligomer. This reaction was conducted in di-oxane at 60° for about 24 hours. Excess piperazine was removed by washing the oligomer extensively with distilled water. Since the piperazine is water soluble and the oligomer is not, the final system was easily purified to very low levels of residual piperazine in this manner. The product was characterized by titration of the amine endgroups. 

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